Electrical power is distributed from central generating plants to homes, offices, and factories as three phase alternating current. It has long been realized that the choice of AC, rather than DC, allows the use of transformers that permit power to be distributed at higher voltages than the voltage at which it is ultimately used. This practice reduces the current in the distribution system and thereby maximizes distribution efficiency by minimizing losses. The frequency of 50 Hz to 60 Hz was chosen as a compromise for wavelength along transmission lines and the size of laminated ferromagnetic core transformers which attain maximum efficiencies well above 90% at 50-60 Hz. To further improve efficiency, and to make high starting torque AC motors simpler to engineer, the power transmission system is three phase.
In recent years there has been an increasing public concern about possible biological effects of the low frequency electrical and magnetic fields associated with the distribution and use of electrical power. One can find, for example, a conjectural review of possible phenomenological bases for such hazards as well as a discussion of a number of recent epidemiological studies in an article by Karen Fitzgerald, et. al, that was published in the August, 1990, issue of the IEEE Spectrum (ISSN 0018-9235). Although the evidence in this area is far from being unequivocally persuasive, the level of public concern has led to litigation aimed at preventing the construction of power transmission lines. The State of Florida has a guideline limiting both electric field strength and magnetic intensity at edge of right of way for new powerlines, Florida Administrative Code Chapter 17-274.
Measurement of both electric field strength and magnetic intensity is made in accordance with ANSI 644-1987 IEEE Standard Procedures for Measurement of Power Frequency Electric and Magnetic Fields from AC Power Lines
In discussing the public health issues related to power lines, it is convenient to consider three classes of power lines:
1) Urban distribution lines running from a substation to distribution transformers located near the point of use, and commonly operated at 10 to 25 kV. PA1 2) Urban transmission lines that supply power to the substations and that are usually not isolated on rights of way and commonly operated at 69 to 138 kV . PA1 3) Rural transmission lines on rights of way and that are commonly operated at voltages above 115 kV.
Although much of the most recent public outcry has been at the third category of power transmission lines listed above, the 10-25 kV category may be more significant because of the vastly greater number of people exposed. Distribution lines in the 10-25 kV category, if mounted on poles, can give rise to measured electrical fields of as much as 20 V/m, and magnetic fields as high as 1.3 micro-tesla when measured on the ground below the lines.
A typical three-phase distribution circuit is four wire, wye connected. A neutral conductor is found in both overhead and buried distribution powerlines. The neutral is used for carrying the unbalanced current as well as for safety purposes. In a dense urban area, the three phase distribution circuit will have on the order of one hundred single phase distribution transformers. These are often connected from phase conductor to neutral. About one third of the total number of transformers is connected to each phase conductor with the object of balancing the load between the three phases and thereby minimizing neutral current. It is well know in the theory of three-phase systems that a balanced load, i.e., a load that has equal current magnitudes and power factor angles for each phase, has zero current in the neutral.
The issue of stray fields from 10-25 kV lines has a well known solution - underground utilities. Both theory and measurements show that placing all three phase conductors in a common steel conduit provides nearly perfect magnetic and electrostatic shielding. There are also aesthetic advantages to underground utilities, which has led to their widespread use in new construction. A major question in the controversy over possible health risks from stray fields is whether or not to rebuild method of reducing or eliminating fringing fields while using the existing wooden poles could provide an economically attractive alternative method of resolving the problem.
In many larger cities in the United States, the second level of electrical distribution is served by 69 kV to 138 kV lines on wooden or steel poles that are located along streets rather than being on dedicated rights of way. It is common to measure electrical fields of over 100 V/M in the front yards of homes that are adjacent to such lines. The magnetic fields associated with these lines, however, are usually lower than those from the lower voltage 10-15 kV distribution lines that are commonly mounted below the 69-138 kV circuit on the same poles.
The intermediate distribution system of category 2 may pose a serious electric field problem. Theoretically, burying the three phase conductors in a common steel conduit, which is commonly used for the 10-25 kV lines, will work at any voltage level, and will eliminate stray electrical fields. If the same wiring geometry is used for these lines as is employed for the 10-25 kV lines, burial will also eliminate stray magnetic fields. Putting all three conductors in a common steel conduit unfortunately poses other problems, partly caused by the difficulty of providing adequate phase-to-phase dielectric insulation at the higher voltages. In addition to the insulation question, the added line capacitance associated with this construction could lead to transmission inefficiencies because of the leading power factor. If, on the other hand, the three phase conductors are buried in separate conduits and the common neutral wye connection is used for the transformers at either end of the line, fringing magnetic fields are not eliminated. In fact, since people walking above the buried line are closer than they would be to a comparable overhead power line, exposure to magnetic fringing fields could be more severe for the underground line.
At larger substations, power from generating plants is supplied at 230 kV or higher. These lines are usually constructed on dedicated rights of way that are typically 60 meters (about 200 ft) wide. For remote power plants and interconnection with other utilities, the powerlines are operated at 345 kV, 500 kV, and 765 kV. These high capacity powerlines are large with steel towers, long insulators, and bundles of conductors. Fortunately, the electrical and magnetic fields from these lines diminish rapidly in strength the further one goes away from the lines and the number of people who live within 200 m of these power lines is small; thus, this category 3 is not as significant a potential public health risk as the other two categories.
One can show from basic physical theory that the strength of electrical and magnetic fields varies dramatically with distance from the two parallel conductors that are commonly used for power distribution. (Magnetic intensity falls inversely with the square of distance.) Near the conductors, the electric and magnetic fields are intense, but once one moves further away from the line than 5 times the conductor-to-conductor spacing, the fields are weak. Although fringing fields can be reduced by close conductor spacing other factors, such as the dielectric strength required to prevent arcing between phase lines, limits the degree of closeness.
The high level of both magnetic fields and electric fields relatively close to the open wire transmission lines is the cause of an occupational health issue related to three phase overhead power lines. Maintenance workers such as tree trimmers receive a significant exposure dose for both electric and magnetic fields, even when working near the 10-25 kV distribution powerlines.
As a practical example, one can consider the significant difference of the fringing magnetic field measured near two types of 120/240 "service drops". In installations made before about 1950, all three wires running from the utility pole to a house were supported on individual insulators. Conductor separation was of the order of 30 cm. As insulating material became more weather resistant, this construction was replaced by a single cable consisting of a support wire and the two "hot" wires which were wrapped around the support wire in a spiral fashion. In this newer design, conductor separation was reduced to about 3 cm. This reduction of conductor-to-conductor spacing reduces fringing fields measured on the ground under the service drop to about 1/10 of the the value for the old wiring technique.
The foregoing discussion is directed toward transmission lines made of several parallel conductors. Another widely used transmission line design is "coaxial", which is widely used when shielding is important. Because of the lower leakage fields at VHF, for example, coaxial cable is replacing parallel line "twin lead" for TV and VCR interconnection and for distribution of cable TV signals. Electrostatic theory indicates that this configuration should have no external electric field. Transmission line theory also shows that there is no magnetic field outside the transmission line if the return current flows in the outer conductor; but, that there is an external magnetic field if some other return path is used. Hence, coaxial cabling offers a wa to eliminate fringing electric and magnetic fields under some circumstances.
It is important to note that if one makes a simple and straightforward substitution of coaxial cabling into a conventional power transmission system, the fringing magnetic fields are not eliminated. For three phase power lines, for example, separate coaxial lines would not eliminate fringing magnetic fields because the return current in any outer conductor would not necessarily be equal and opposite to the source current in the associated central conductor. Some reduction in magnetic fields caused by return current shielding would be expected. In addition, with the elimination of "arcing" between conductors caused by line to line voltage, separate coaxial cables could be located very close together and therefore markedly reduce the fringing fields. It should be noted that all of the coaxial cable methods would eliminate fringing electric fields.
This defines the problem to be solved by my invention: Eliminate the fringing magnetic fields as well as the electric fields of a three phase power transmission line.